This invention relates to holography, to elements for holographic recording and display, and more particularly to the manufacture of large, nonplanar elements, termed "pre-holographic" in that they are easily treated with a sensitizing solution to allow them to record and display volume phase holograms.
Recording of holograms in the form of a three-dimensional phase grating or relief pattern in an optical-quality medium is well known. Coherent light is split into an object beam and a reference beam. The object beam is passed through the object image to be recorded and is deflected to intersect the reference beam at the surface of a photosensitive medium. A complex interference pattern is created, caused by a spatial variation in the index of refraction of the medium. When properly illuminated the halogram will display a visual image of the recorded object. Other imaging applications of the media are well known to those skilled in the art.
A typical recording medium comprises a transparent base coated with a photosensitive layer. It frequently takes the form of tape or microfilm with a base of, for example, poly(ethylene terephthalate) resin or acetate resin; numerous commercially available photoresist materials may be used on these bases to form the photosensitive layer.
Gelatin recording media are desirable as photosensitive layers because, inter alia, they produce a hologram with a high diffraction efficiency; however, to date, they have until recently been employed on only a limited number of bases, mainly glass, in holographic applications.
Photosensitive layers containing gelatin have been used in photography on a number of film bases or substrates. In the case of a hydrophobic substrate, it has been necessary to coat the substrate with one or more, sometimes as many as four, subbing layers in the form of a suitable colloid dissolved in organic solvents such as methanol, ethanol, acetone, n-butyl acetate, or appropriate mixtures of these and other solvents. The colloids include, for example, homo-, co-, and terpolymers of the alkyl acrylates, alkyl methacrylates, unsaturated polyesters, and chlorosulfonated poly(ethylene).
Regenerated cellulose has been used as a subbing layer for gelatin on cellulose acetate film bases. This was accomplished by a superficial saponification of the cellulose ester. Poly(methyl methacrylate) is extremely resistant to mild saponification.
Poly(methyl methacrylate) sheet, more commonly referred to as PLEXIGLAS, a trademark of the Rohm and Haas Co., has good optical properties but has not, until recently, been used with gelatin, either as a substrate in a photographic application or as a support in a holographic application. Poly(methyl methacrylate) is probably less attractive as a holographic base in microfilm applications because it is less flexible and more brittle in thin sheets than many other polymers. It is also highly hydrophobic and, therefore, difficult to bond to gelatin.
It has recently been reported by the inventors, in U.S. Pat. No. 3,928,108, that a hardened but swollen and flexible film of gelatin may be attached to a curved piece of poly(methyl methacrylate) which has been subbed by a two-step operation in which the support is coated with nitrocellulose and the nitrocellulose is denitrated with an ammonium sulfide and ethanol solution. However, the hardened gelatin film is quite fragile and easily scratched and large films cannot readily be transferred to large poly(methyl methacrylate) supports.